Bearings for use in negative-pressure environments

ABSTRACT

A vacuum pump includes a rotor and at least one bearing that is supplied with a lubricant and that is situated, during operation, in a negative-pressure environment. In order to extend the useful life of the bearing, it is proposed that the bearing is formed from a ceramic material.

This is a continuation of application Ser. No. 396,514, filed Aug. 21,1989 now U.S. Pat. No. 5,028,219.

TECHNICAL FIELD

The invention is directed to a vacuum pump that includes a rotor and atleast one bearing that is supplied with a lubricant and that issituated, during operation, in a negative-pressure environment.

BACKGROUND OF THE INVENTION

Lubrication of bearings that are subject to a negative-pressureenvironment during operation is problematical, since the lubricantsgenerally employed are not vacuum-resistant over the long term andgradually volatilize. Such volatilization occurs regardless of whetherlubricating greases or lubricating oils are used and thus lubrication ofthe contact members of the bearings is not always guaranteed. Oneconsequence of a lack of lubrication is intensified frictional phenomenaor "cold welding" between the rolling members and bearing rings, whichleads to increased bearing wear and, thus, to a reduced useful life forthe bearing. When bearings operate in a vacuum, it is impossible toavoid a lubricant deficiency merely by supplying large amounts oflubricant, because excessive quantities of lubricant lead to anincreased formation of lubricant vapor. If these lubricant vaporsproceed into the vacuum produced by the pump, they can deteriorate oreven contaminate pump components. Moreover, the rotors of vacuum pumpsare frequently operated at extremely high speeds, so that excessquantities of lubricant lead to an additional bearing stress. The abovedescribed problems are of particular significance in turbomolecularvacuum pumps, whose rotors are operated at 60,000 revolutions per minuteand above. Attempts have been previously made to resolve these problemsby precisely metering the quantity of lubricant supplied to thebearings. Examples of such solutions are disclosed in German PublishedApplications 21 19 857, 23 09 665 and 29 47 066.

SUMMARY OF THE INVENTION

A primary object of the present invention is to equip a vacuum pump, ofthe type including a rotor and at least one bearing t hat operates in anegative-pressure environment, with a bearing that is significantlybetter suited for operation in a vacuum.

This and other objects are inventively achieved in that the bearing orbearings of the vacuum pump that operate in a negative-pressure sectionof the pump are ceramic bearings. One advantage of using ceramicbearings is that their operational temperatures are low in comparison tosteel bearings previously employed. This means that less lubricantvolatilizes, so that the risk of interrupting the minimum lubricant filmbetween contact members of a rapidly rotating vacuum pump issignificantly lower. Despite lesser quantities of Iubricant, adequatelubrication of the bearings can be more likely ensured than with steelbearings. Moreover, "cold welding" can no longer occur, and thus theincreased frictional and sliding phenomena, increased bearingtemperatures, and increased bearing wear common with steel bearings areavoided. The ceramic bearings of the present invention achieveconsiderable extension of the useful life of the bearings.

In addition to the advantages set forth hereinabove, the ceramic bearingforms an electrical insulation between the rotor and the housing of thevacuum pump. EIectrical currents that may lead to bearing damage cantherefore no longer flow through the bearing. Such currents arise when avacuum pump, particularly a turbomolecular pump, is operated in amagnetic field; for example, in nuclear power plants or in the proximityof magnets of accelerators.

Yet another advantage of the present invention lies in that reliablebearing lubrication allows the vacuum pumps to be operated at higherspeeds, which results in an improvement of the pump performanceparticularly of the pumping capacity.

In a preferred embodiment of the present invention, the bearing operatedin a negative-pressure environment is a rolling bearing includingrolling members and inside and outside bearing rings. Only the rollingmembers are composed of ceramic. Since ceramic rolling members arelighter in weight than steel rolling members, the centrifugal forcesacting on the bearings and, thus, the inner bearing stresses are lower.Consequently, lower setting forces are required in spindle bearings forsetting a defined thrust angle. Reduced settling forces result in areduction in the surface pressure and of the frictional forces generatedby the bearings during pump operation. Fluctuations in thrust angle thatare harmful, particularly at extremely high speeds, are also reduced.

Further advantages and details of the invention will become apparentupon reference to the accompanying description, when taken inconjunction with the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial section through a turbomolecular vacuum pumpembodying the present invention.

FIG. 2 is a section through a bearing spindle in a second embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a section through the lower part of a turbomolecular vacuumpump including a pump housing 1, a stator 3 equipped with stator paddles2, and a rotor 4 having rotor paddles 5. A shaft 6 having an axis 7 is acomponent part of the rotor 4. The rotor paddles 5 are secured to theshaft 6. The stator paddles 2 and the rotor paddles 5 are set such thata gas is conveyed from a pump inlet (not shown) to a pump dischargeorifice 8.

The rotor shaft 6 is rotatably seated in the housing 1 of theturbomolecular pump by means of rolling bearings 11 and 12. A drivemotor 13 is situated between these bearings. Since a backing pump isconnected to the discharge orifice 8, the space in which the bearings11, 12 and the motor 13 are situated is subject to negative pressureduring the operation of the pump.

The bearings 11, 12 are fashioned as ceramic bearings, i.e. at least therolling members 14 of these two bearings--preferably balls--are composedof ceramic material. It has been found that ceramic componentsmanufactured using a hot-pressing process or an isostatic pressingprocess, preferably likewise under heat, are especially suited for usein rolling bearings because of the surface properties such processesimpart.

The rolling bearings 11, 12 are oil-lubricated. An oil circulationsystem is provided that includes an oil duct system 15, 16 Oil issupplied to this duct system in metered fashion by an oil conveying pump17, with the oil being supplied from an oil reservoir 18. Excess oilflows back into the reservoir 18 through an oil return duct 19.

Due to the provision of ceramic bearings, the quantity of oil suppliedto the bearings can be kept extremely small compared to the quantityrequired for conventional bearings. The bearing temperature remains lowduring the operation of the pump, and the volatilizing quantity of oilis therefore slight. Even if interruption of the lubricant film shouldoccur, the wear phenomena that significantly shorten the useful life ofconventional steel bearings do not occur, since ceramic bearings haveespecially good emergency running properties.

The bearing spindle shown in FIG. 2 that is suitable for employment invacuum pumps includes spindle bearings 11, 12 that are fashioned asceramic bearings. The inside rings of the bearings 11, 12 are supportedon shoulders 21, 22 of the shaft 6. The respective outside rings pressagainst a spindle sleeve 23 in radial direction. In axial direction,they are supported on sleeves 24, 25. The latter are guided in thehousing or spindle sleeve 23. They are under the influence of acompression spring 26 that generates the setting forces for thebearings. The system composed of the shaft 6, the bearings 11, 12, thesleeves 24, 25 and the spring 26 is fixed inside the spindle sleeve 23with the assistance of a ring 27 and of a screw 28.

The end-face termination of the spindle sleeve 23 is formed by caps 28and 29 that are fixed with clamp rings 31, 32. Largely closed, annulargrease reservoirs 3S, 34, situated in the caps 28, 29 assure long-termlubrication of the bearings 11, 12, due to the decreased demand forlubricant. The end faces of the sleeves 24, 25 that face toward thebearings 11, 12 include similar grease reservoirs 36, 37.

A particular advantage of fashioning the bearing 11, 12 as ceramicbearings is that the centrifugal forces generated by the ceramic rollingmembers ar lower than the centrifugal forces generated by steel rollingmembers. It is therefore possible to keep the setting forces for thebearings lower, i.e. to select the compression spring 26 so that itsbiasing force is weaker. The friction generated by the rolling memberson the bearing rings is therefore lower, which results in an extensionof the useful life of the bearings.

The present invention is particularly advantageous when alubricant-supplied rolling bearing must be operated in anegative-pressure environment. Mineral oils or synthetic oils, oils freeof hydrocarbons (for example, perfluoridated polyether), or greases, canbe utilized as lubricants in pumps. Lubricants free of hydrocarbons areparticularly useful in vacuum technology, when the gas to be conveyed orwhen the volume to be evacuated must be kept completely free ofhydrocarbons. Compared to other lubricants, perfluoridated polyethershave poorer lubricating properties because, among other things, theylead to the formation of iron fluoride, which increases the risk ofruptures in the lubricating film between bearing contact members. As aconsequence of the use of ceramic bearings in the present invention,such ruptures in the lubricating film no longer necessarily lead topremature bearing damage and are thus of lesser concern. When theceramic bearings are fashioned as rolling bearings, then at least therolling members should be composed of ceramic. With a bearing fashionedas a bearing spindle (see FIG. 2), again, at least the rolling membersshould be composed of ceramic. A separate inside bearing ring is notrequired when the spindle is integral with the shaft, i.e. when theshaft itself is equipped with the grooves that form the ball races. Withany type of bearing employing the present invention, at least one of thebearing contact members must be composed of ceramic.

Although the present invention has been described wit reference toparticular embodiments, those of ordinary skill in the art willrecognize that various changes may be made thereto without departingfrom the scope and spirit of the invention as defined by the appendedclaims.

We claim:
 1. A vacuum pump of the type including a rotor and at leastone bearing supporting said rotor, said vacuum pump comprising thefollowing:said at least one bearing including a plurality of ceramicrolling members, an inside bearing ring, and an outside bearing ringlocated in an environment subject to negative pressure during operationof aid pump; means for supplying lubricant to aid at least one bearingand establishing a lubricant film between said rolling members and saidbearing rings; setting means including at least one setting sleeveaxially reciprocable in a housing a compression spring biasing said atleast one setting sleeve at least one cap closing said housing, andlubricant reservoir means, within said setting means for supplyinglubricant to aid at lest one bearing.
 2. A vacuum pump of the typeincluding a rotor and at least one bearing supporting said rotor ofclaim 1, wherein said lubricant reservoir means comprises at least onelubricant reservoir disposed in said at least one setting sleeve.
 3. Avacuum pump o the type including a rotor and at least one bearingsupporting said rotor of claim 1, wherein said lubricant reservoir meanscomprises at least one lubricant reservoir disposed in said at least onecap.
 4. A vacuum pump of the type including a rotor and at least onebearing supporting said rotor of claim 2, wherein said lubricantreservoir means comprises at least one lubrican reservoir disposed insaid at least one cap.
 5. A vacuum pump according to claim 1 furtherwherein said at least one bearing is a spindle bearing, and said rollingmembers comprises a plurality of balls.
 6. A vacuum pump according toclaim 5 further wherein said lubricant is perfluoridated polyether.